The Securities and Exchange Commission probe of shares trading on private marketplaces might result in pressure for Facebook to go public.

The Securities and Exchange Commission’s probe into trading of Facebook shares on private marketplaces has some asking whether the regulator might pressure the social network to have an initial public offering.

An article in The New York Times says the SEC gave Facebook permission to grant restricted stock to its employees without having to list them publicly and make the financial disclosures required of public companies.

Federal law draws the line between public and private at 500 shareholders. Facebook obviously has more than twice that number of employees, but the SEC might be concerned with how many stakeholders aren’t employed by the social network.

The auctions on SecondMarket and SharesPost of Facebook holdings are currently limited to professional investors. But new funds that own some of these shares potentially give access to a wider array of buyers.

Even so, these funds still have some prerequisites about the minimum net worth who can buy in. That combined with the rules of SecondMarket and SharesPost could conceivably slow down the growth in the number of shareholders to a level acceptable to the SEC.

Or maybe not. That’s why the regulator’s asking questions right now. Perhaps the SEC’s timing will complement other legal developments so that any pressure for a Facebook IPO would kick in after the Winklevoss twins go away.

Oral arguments for the twins’ appeal of the $65 million settlement begin next month, and the issue going before the court concerns allegations of securities fraud. Those are two words that no one wants to see in an IPO prospectus.

Do you think the SEC’s investigation of Facebook stock trades on private markets will lead to regulatory pressure for an IPO?